1. Field of the Invention
This invention relates to a rotation sensor for vehicles, and more particularly to a rotation sensor for vehicles used for a speed meter, a tachometer or a trip meter, or the like.
2. Description of the Prior Art
A conventional rotation sensor for vehicles has such a construction as described in FIG. 1, which shows the construction around a drive shaft of the sensor.
In the drawing, the numeral 1 is a housing of the sensor which has a cylindrical bore 2 to which a drive shaft 5 is inserted and an enlarged cylindrical portion 3 formed in the bore 2. A pair of bearings 4 which support both sides 5a and 5b of the drive shaft 5 rotationally are provided on the edge portions 3a of the enlarged cylindrical portion 3, respectively. The drive shaft 5 is connected to a rotational shaft of an engine or a transmission of a vehicle (not shown) by means of a transmission member 6 which is fixedly connected to a connecting section 5c formed in the drive shaft 5. Said connecting section 5c has, as shown in FIG. 2C, a square shape in cross section which is in tight engagement with an end portion of the transmission member 6 which also has a square shape in cross section fittable to the connecting section, so that the rotational force of the engine or the like is transmitted to the drive shaft 5 securely.
In the middle portion of the drive shaft 5, there is formed a ring-shaped enlarged diameter portion 7 integrally, which is located between the bearings 4. On the outer circumferential surface of the enlarged diameter portion 7, there is provided a circumferential recess 7a, on the other hand there are provided notches 7b on the diametrical opposite sides of the ring-shaped enlarged diameter portion 7, as shown in FIG. 2B.
Around the ring-shaped enlarged diameter portion 7, there is formed a magnetic member 8 by insert molding (insert molding is a name of a process fixing a resin to a part of metal material by extrusion). In this case, by the provisions of the circumferential recess 7a, the fixing force between the ring-shaped enlarged diameter portion 7 and the magnetic member 8 can be secured in the axial direction, and by the notches 7b the fixing force in the rotational direction can be also secured.
Said magnetic member 8 comprises, as shown in FIGS. 1 and 2A, a ring-shaped supporting member 8a of a synthetic resin which has a recess 8b on its outer circumferential surface, and a ring-shaped ferrite magnet 8c magnetized to multipoles which is provided in the recess 8b. Said magnetic member 8 is rotated within the enlarged cylindrical portion 3 in accordance with the rotations of the drive shaft 5.
On the inner circumferential surface of the enlarged cylindrical portion 3 at the opposite position to the ring-shaped ferrite magnet 8c, there is provided a detector 9, such as an induction coil or magnetic sensitive element (e.g. Hall element or magnetic resistance element) and so on, for detecting changes in magnetic flux which is caused by the rotations of the ring-shaped ferrite magnet 8c, thereby the rotational frequency and rotation speed can be measured.
In the meantime, it is required for these rotation sensors to be connected to a rotational shaft of an engine or a transmission with a short transmission member in proximity to the engine or the transmission for obtaining a stability in operation, miniaturizing its size and lightening its weight. However, in the rotation sensor having conventional structure described above, there is a problem that the sensor can not be equipped in proximity to the engine around which a temperature rises more than 130.degree. C., since the ring-shaped supporting member 8a of the magnetic member 8 is insufficient in heat resisting property due to being commonly formed of a synthetic resin such as polyacetal resin or P.B.T. (Polybutylene Terephthalate).
In order to solve the problem, it was proposed to form the magnetic member 8 of a composition of a heat-resisting synthetic resin such as polyphenylene sulfide and ferrite particles, and to attach thus formed magnetic member 8 around the ring-shaped enlarged diameter portion 7 of the drive shaft 5, as shown in FIG. 3. However, although the magnetic member 8 thus formed has a good heat resisting property, it is liable to cause a crack in the magnetic member 8 since the magnetic member 8 is expanded or contracted repeatedly due to changes of a temperature within the extent over 130.degree. C. around the engine.
In order to prevent the occurrence of the crack, it was proposed that a ratio of b/a ("a" is an inner diameter of the magnetic member 8 and "b" is an outer diameter of the magnetic member 8) is set to more than e.g. "b .gtoreq.10a", or heat expansion coefficiency of the synthetic resin contained in the magnetic member 8 is set to equal to that of the materials of the drive shaft 5. However, in the former way, there are disadvantages that a fixing force of the magnetic member 8 to the drive shaft 5 is deteriorated due to decrease of the inner diameter "a" and a size of the magnetic member becomes larger due to enlargement of the outer diameter of the magnetic member 8, while in the later way there is a disadvantage that a cost of the manufacturing is increased due to a price of the synthetic resin being high. In addition, when the magnetic member 8 is formed by insert molding, it is necessary to determine a structure of the die of the magnetic member and a molding condition strictly to prevent an occurrence of a weld line which is one of the factors which causes the crack, so that there is a disadvantage that a manufacturing cost is also increased.
Further, these disadvantages result in a trouble which causes a falling off of the magnetic member from the drive shaft.